1. Field of the Invention
The present invention relates to a ring oscillator and, more particularly, to a ring oscillator capable of operating under a low-voltage condition, while preserving excellent operating speed and linearity.
2. Description of Related Art
In recent years, the issue of environmental protection and energy efficiency is getting more and more important. For electronic products, it can be discussed in two aspects. First, for many kinds of electronic products, a battery is the main source for power supply, and thus to extend the lifetime of the battery is considered important. Second, with the technique of low-power circuit design, it is desirable to allow the circuit to maintain the efficiency that the circuit should meet under low-power consumption. Thus, the second aspect is equivalent to the effect of extending the lifetime of the battery. Accordingly, low-power circuit design is important to many current electronic devices with high precision, such as handheld telecommunication products or PDAs.
Furthermore, combining multiple integrated circuits (ICs) of different functions on a single chip can now be done due to advanced semiconductor processes. This is the concept of system on chip (SoC) substituting for the traditional printed circuit board (PCB), and thus the volume and weight of electronic products are decreased.
It is known that the power consumption is related to the operating frequency, the load capacitance, and the operating voltage pursuant to theory of P=fCV2, where P represents the power consumption; f represents the operating frequency; C represents the load capacitance and V represents the operating voltage. Thus, the most effective way is to reduce the operating voltage of the circuit if it is desired to reduce the power consumption of the circuit. Reducing the operating voltage allows the power consumption of the circuit to decrease correspondingly; even operating the circuit within the sub-threshold region is able to be done.
The threshold voltage (VTH) of the transistor becomes the most critical issue while being operated under low-voltage operation. Due to the development of the semiconductor process, the operating voltage of circuits keeps decreasing, but the decreasing breadth on the threshold voltage of transistors is unable to meet the operating voltage. When the operating voltage decreases, the potential difference between gate and source of the transistor decreases as well, which causes the reduction of the driving current; whereby the time of charging and discharging the entire digital circuit also increases, and the operating speed of the circuit is lowered. This is shown in FIG. 1, in which the gate delay of the transistor increases dramatically when the operating voltage decreases. In addition to the operating speed on circuit, the effect on the entire system caused by process, voltage and temperature (PVT) variation is greater than while being operated under regular operating voltage. Therefore, process drift becomes very severe.
As described above, there are two main issues to confront when a circuit is operated under low-voltage operation. The first one is the degradation of driving ability; when the operating voltage decreases, the potential difference between gate and source of the transistor decreases as well, causing the operating speed of the circuit to slow down. For the second one, when the operating voltage of the circuit is in the sub-threshold region or in the neighbor of sub-threshold region, the effect on the system caused by process voltage temperature (PVT) variation is greater than that operated under regular operating voltage.
Therefore, it is desired to have an oscillator circuit which is capable of operating under low-voltage condition while preserving excellent operating speed and linearity.